The present invention relates, in general, to communication systems and, more particularly, to coding information signals in such communication systems.
Code-division multiple access (CDMA) communication systems are well known. One exemplary CDMA communication system is the so-called IS-95 which is defined for use in North America by the Telecommunications Industry Association (TIA). For more information on IS-95, see TIA/EIA/IS-95, Mobile Station-Base-station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System, March 1995, published by the Electronic Industries Association (EIA), 2001 Eye Street, N.W., Washington, D.C. 20006. A variable rate speech codec, and specifically Code Excited Linear Prediction (CELP) codec, for use in communication systems compatible with IS-95 is defined in the document known as IS-127 and titled Enhanced Variable Rate Codec, Speech Service Option 3 for Wideband Spread Spectrum Digital Systems, January 1997. IS-127 is also published by the Electronic Industries Association (EIA), 2001 Eye Street, N.W., Washington, D.C. 20006.
In modern CELP coders, there is a problem with maintaining high quality speech reproduction at low bit rates. The problem originates since there are too few bits available to appropriately model the xe2x80x9cexcitationxe2x80x9d sequence or xe2x80x9ccodevectorxe2x80x9d which is used as the stimulus to the CELP synthesizer. One common method which has been implemented to overcome this problem is to differentiate between voiced and unvoiced speech synthesis models. However, this prior art suffers from problems as well. Thus, a need exists for an improved method and apparatus which overcomes the deficiencies of the prior art.